This invention relates to AC-DC power converters and, in particular, to AC-DC power converters having included therein a bootstrapped control system that prevents the input current from exceeding acceptable levels.
Conventional 50/60 Hz input power supplies are large and heavy due to the input isolation transformer and the filter for the rectified power line. One method used to reduce the size of the input isolation transformer is to use the off-line switching regulation technique, wherein the input voltage is rectified and filtered directly and then passed through a pulse width modulated inverter which includes an isolation transformer. The transformer operates at the inverter switching frequency, typically 20/50 kHz and is thus a magnitude smaller then the typical brute force type AC-DC 50/60 Hz inverter transformer. Further size reduction in the inverter components is possible using the higher switching frequencies of 200/500 kHz, made feasible by power MOSFETS. However, even at the higher kHz switching frequencies, the size of the power supply is dominated by the filter capacitor for the rectified power line.
FIG. 1 is a simplified block diagram of a prior art AC-DC power converter system 10 that utilizes the off-line switching method of regulation. A voltage source 1 provides an AC voltage signal to a full wave rectifier 3 where the AC voltage is converted to a rectified AC voltage signal. The output of the full wave rectifier is applied to a filter capacitor 5 for removal of the ripple components and to a switching mode power supply 7 where the rectified AC voltage signal is converted to a DC signal for application to a load 9 across which the voltage V.sub.o is developed and is indicated by the dimension lines 11. The input current I.sub.1 to the switching mode power supply 7 is represented by the flow arrow 13. In the situation where the above-referenced circuit is used for power supplies that deliver high power, then the size of the capacitor 5 must be extremely large. Otherwise, the voltage V.sub.1 across the capacitor 5 drops below acceptable levels under high current conditions.
In FIG. 2, there is a curve 15 illustrating the rectified voltage V.sub.1 where there is no filter capacitor. The output voltage that is the voltage applied across the resistor 9 is designated as V.sub.o and is represented by the dash line 17. The current I.sub.1 is represented by the curves 19. When the voltage V.sub.1 that is on the output of the rectifier 3 approaches zero, as indicated at point 21, the current that is represented by the curve 19 approaches infinity. This, of course, is an undesirable condition for the operation of an AC-DC converter.